What Just Happened?

My girlfriend and I split up during the spring. We had dated for three years during college. It was a serious relationship that I thought would last forever. I wanted it to. After the split, communication between us was minimal. I would send texts and emails occasionally professing my feelings, etc. Fast forward to this past summer and things seemed liked they were looking up. We were communicating more frequently and talking about things that we could improve if we were to get back together. One weekend while she was at home with her parents (which was four hours away from my location), we planned to go camping and spend some quality time together. We were to talk about another potential future together. That weekend came and I drove the four hours to see her. I was excited at the idea of talking things out and getting back with her. The first day and night we set up camp and did things together; everything seemed great. It was like old times. The next day we were planning an activity. I took a shower and once I finished she said that I should go home. It caught me off guard as I thought things were looking up. I drove four hours home flabbergasted. She wouldn’t return my calls and I discovered she blocked me from Facebook. All I wanted was a clear explanation. I soon found out that my best friend knew that she was also seeing someone else on the side that I did not know about.